1. Field of the Invention
The present invention relates generally to pouring tubes for use with continuous strip-casting equipment. More particularly, the invention relates to pouring tubes which are designed for the introduction of a metallic melt into the cavity of the mold through lateral discharge ports located below the normal operational surface level of melt in the mold.
2. Summary of the Prior Art
In the continuous casting of metallic strip, particularly steel or aluminum strip, it is well understood that melt must be supplied to the cavity of the mold in sufficent quantity to allow the maintenance of efficient casting speeds. It is also well understood that the non-uniform distribution of melt within the cavity of the mold can detrimentally effect the quality of the strip being cast. The introduction of desirable quantities of melt into the cavity of the mold in a manner which maintains the required uniformity of distribution of melt within the mold, however, has heretofore presented serious problems to the art. For example, the so called "free-pouring" method has been found to be unsuitable for the continuous casting of steel strip. This method results in a melt distribution within the mold which is not uniform. Accordingly, acceptable product quality cannot be assured. Further, the "free pouring" method introduces unacceptable heat loss and re-oxidation problems in the continuous casting context.
Attempts have been made to solve this problem by introducing melt below the normal operational melt surface level in a continuous strip-casting mold. For this purpose, molds for the continuous casting of metallic strip have been developed having an enlarged pouring area which tapers inwardly and downwardly from the to of the mold centrally between the broad side walls and the narrow side walls which form the cavity of the mold. Pouring tubes for use with these molds have also been developed. These pouring tubes extend from a melt container to the mold cavity, and include a discharge portion (adjacent to their free ends) which extends into the volume of the enlarged pouring area below the normal operational melt surface level in the mold. The discharge portion of such pouring tubes is closed axially, and includes a lateral exit port facing each of the narrow side walls of the mold. The melt flows from the container through the pouring tube to its free end. At the free end of the tube, the melt is deflected outwardly through the lateral exit ports, toward the narrow side walls of the mold. The resulting melt stream flow into the melt already present in the mold cavity radially from the pouring tube toward the inner surfaces of the narrow side walls of the mold. Since the melt in each of these streams has a high kinetic energy (i.e., the flow is fast in order to assure that an adequate quantity of melt for efficient strip casting reaches the cavity of the mold), the identifiable currents created by the inflooding melt within the melt already present in the cavity extend from the pouring tube all the way to the walls of the mold. This results in erosion of the strand shell under formation along the walls of the cavity. The presence of an appropriate strand shell is important to the casting of strip having acceptable structure and a uniform, fault free surface. Hence, the damage to (and/or the impediment to the formation of) the strand shell in such prior art devices must, if possible, be strictly controlled. This means that melt introduction rates must be held below optimum levels, thus reducing the efficiency of the entire casting operation.
Similarly, since the volume of melt normally present within the cavity of the mold is not large (particularly in cases wherein strip having thin cross-sectional height, for example, less than 60 mm, is being cast), and since that volume cannot readily absorb the energy of the incoming melt streams, the streams flowing outwardly from the discharge ports towards the narrow side walls also tend to create non uniform distributions of melt within the cavity of the mold, i.e., an excess of melt (standing waves) adjacent the narrow side walls of the cavity. This excess melt is forced upwardly (adjacent the narrow side walls) because it is confined by the broad side walls, a narrow side wall, the melt below it, and the incoming stream of melt behind it. Therefore, the surface of the melt in the cavity is raised adjacent each of the narrow side walls relative to its normal level, creating a trough in the area adjacent the pouring tube. This can cause deviations from the preferred flow characteristics of the melt through the mold which are detrimental to the formation of acceptable strip. It also may cause metal to become caked on the walls of the mold above the normal level of the melt therein. In extreme cases, the energy of the streams may be great enough to cause splashing of the melt adjacent the narrow side walls of the mold. Melt splashing is particularly problematic during the start-up phase, when the melt in the cavity of the mold has not yet had a chance to reach its normal operational level.